Qt: passing objects to every window - c++

Question

Im a beginner at C++ since I'm a Java-Dev. I'm learning Qt right now.
Now I want in my widget application on central object for holding special functionality (for example a bunch of methods for sending different network-command or something like that). In my mainwindow.cpp I've got this
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
//...
BusinessLogic* bl = new BusinessLogic;
bl->setMyValue(44);
this->setBusinessLogic(bl);
this->getBusinessLogic();
//...
}
BusinessLogic* MainWindow::getBusinessLogic()
{
return this->bl;
}
void MainWindow::setBusinessLogic(BusinessLogic* newbl)
{
this->bl = newbl;
}
Where BusinessLogic.cpp is the class which I want to access from all windows in my application. So in my Dialog which get opend when the pushButton is clicked, I try to use the businesslogic instance like this in the constructor
Dialog::Dialog(QDialog *parent) :
QDialog(parent),
ui(new Ui::Dialog)
{
//...
((MainWindow*)parent)->getBusinessLogic();
}
Unfortunately the program crashes everytime the dialog get instanciated (the line with ((MainWindow*)parent)->getBusinessLogic(); get called.
What can I do?

Answer 1

There are several methods for achieving your goal(all depends on your requirements) but one approach is you can create Singleton class of your BusinessLogic and then access it anywhere. have a look here. OR you can also pass object of class one class to another. OR you can use signal slot mechanism.
For you best Solution can be
Class BLogic{
public:
static BLogic* instance(){
static BLogic ins;
return &ins;
}
void setVal(int v){
val = v;
}
private:
BLogic(){}
int val;
};
Now you can call functions of BLogic from any class like
BLogic::instance()->setVal(10);
OR Example of passing objects can be
class ClassA
{
public:
void setClassB(ClassB *ob){
m_classB = ob;
}
ClassB* getClassB(){
return m_classB;
}
private:
ClassB* m_classB;
};
class ClassB
{
public:
void setClassA(ClassA *ob){
m_classA = ob;
}
ClassA* getClassA(){
return m_classA;
}
private:
ClassA* m_classA;
};
Now from main.cpp
ClassA* obA = new ClassA;
ClassB* obB = new ClassB;
obA->setClassB(obB);
obB->setClassA(obA);

Answer 2

Don't pass a zero parent or a dangling pointer to the dialog.
But more seriously, verify your assumptions:
qDebug() << "the parent object is" << parent;
auto p = qobject_cast<MainWindow*>(parent);
if (p)
p->getBusinessLogic();
Debug output will give you some idea of whether a parent is set, and of what type it is at the moment. Remember that during the construction of a MainWindow instance, it goes through being different types (quite literally). So depending on when you construct the child, you might get a parent of type QWidget* or QMainWindow* or MainWindow*. During the destruction you could even get a QObject* parent!